In May, at the end of another warm Australian summer, Queensland-based marine biologist Dr James Kerry escorted a friend out on a scuba dive trip to the Great Barrier Reef.

As one of the reef's preeminent scientists, Dr Kerry was much better prepared than his dive buddy for what they were likely to encounter at the prized UNESCO World Heritage site. That day, entering the water, imprinted in the back of his mind were haunting visuals from a series of exhaustive, probing flights he had conducted over the reef in 2016 and 2017.

Those flights, which clocked days of fly time and covered thousands of kilometres, were part of a wider scientific investigation into how much damage those two major back-to-back coral bleaching events had inflicted on the reef. He and a fellow reef researcher on board the plane had been shocked and upset by what they had seen. Huge tracts of the reef, the world's largest living organism, had been bleached.

In fact, Dr Kerry and scientists from the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Townsville concluded two-thirds of the Great Barrier Reef had suffered mass coral bleaching.

Last month, after finishing their May dive on a section of the reef which had been bleached and damaged, it was the turn of Dr Kerry's diving friend to be surprised.

"He said it was like a funeral," Dr Kerry told nine.com.au.

"Maybe he was being a bit emotive, but I knew what he meant because it is very quiet on the reef," he added.

In 2016, bleaching was most severe in the northern third of the Reef, while one year on, the middle third has experienced the most intense coral bleaching. (Graphic: ARC Centre of Excellence for Coral Reef Studies) ()

In the six months following the peak of bleaching in March 2016, he and fellow marine scientists measured on average 67 percent loss of corals in the northern 700 km section of the Great Barrier Reef. Of all the sections of the 2300km-long Great Barrier Reef, the northern third had always been the most pristine.

Last year, the middle third of the reef sustained mass bleaching.

"The reef is struggling," Dr Kerry said.

"There has been two-thirds of the reef impacted in two years, and where there was an overlap of bleaching there has been no chance for recovery.

A reef researcher dives to observe the impact of mass bleaching on the Great Barrier Reef in 2017. In 2016, bleaching was most severe in the northern third of the Reef, while one year on, the middle third has experienced the most intense coral bleaching. (Photo: ARC Centre of Excellence for Coral Reef Studies) ()Researchers from ARC Centre of Excellence for Coral Reef Studies said bleaching is caused by record-breaking temperatures driven by global warming. (Photo: ARC Centre of Excellence for Coral Reef Studies) ()

"That has changed the face of the reef for a very, very long time into the future."

In recent decades, there have been four major bleaching events on the Great Barrier Reef: 1998, 2002, 2016 and 2017.

When bleaching occurs, colourful marine algae, known as zooxanthellae, which lives inside coral, dies. This causes corals to turn white, and bleach͛.

Dr Kerry and the ARC Centre of Excellence for Coral Reef Studies are clear about what they believe has caused the bleaching: climate change. The rise in global temperature has caused warmer waters, which has then bleached the thermally-sensitive Great Barrier Reef, according to Dr Kerry.

Bleached corals can recover if the temperature drops and life-giving zooxanthellae is able to recolonise them. However, back-to-back bleaching events significantly affect the reef's ability to regenerate the wondrous kaleidoscope of colour, and the marine life it supports.

"What the Great Barrier Reef has going for it is its size," he said. "But it would be naive to think we won't have another bleaching event in the next ten years."

The best-case scenario is one bleaching event in the next decade, according to Dr Kerry.

On the other hand, he is fearful consecutive or multiple major bleaching could strike in a short period of time. "If they were at the scale of [2016 and 2017] I don't think the reef would look the same possibly ever again,” he said.

Dr Kerry said it is a difficult time to be a reef researcher because most of the news emerging from the ARC Centre of Excellence for Coral Reef Studies team is negative. In 2016 and 2017, when he was on the small plane, he surveyed around 1000 of the 2900 reefs that make up the magnificent marine ecosystem.

"There were days when we literally didn't see a reef that wasn't bleached - and we were flying over hundreds of reefs a day," he said. "That was quite sobering, to realise the kind of damage that is occurring under the water."

At the same time he was up in the sky, teams of divers milled underwater, gathering data on the reef. He said divers were finding some reefs like "graveyards".

"You had to really swim to find a living coral. That was upsetting," he said.

Dr Kerry is staggered that plans to open the controversial and gargantuan Adani coalmine remain alive.

Located in the Galilee Basin in central Queensland, the $16.5 billion mega Adani mine is calculated to extract and export 27 million tonnes of coal a year.

"Why are we trying to open up the biggest coalmine in the world?" he said.

"To bring that coal out of the ground, and to pump that carbon into the atmosphere would be reckless when there is so much capacity in Australia for renewable energy."

Queensland's Minister for Environment and the Great Barrier Reef Leeanne Enoch said in a statement: "We know that the number one challenge facing the reef right now is climate change."
She said protecting the Great Barrier is a key priority for the Palaszczuk government, and that the proposed mine had undergone "extensive and rigorous approval processes" at state and commonwealth levels.

Earlier this month the Queensland government allocated a record $330m over five years to protect the reef.

Dr Kerry said studies carried out after the 2017/18 Australian summer showed no signs of bleaching. "It was a great reprieve," he said.